1. Field of the Invention
The present invention relates to a solid-state image pick-up apparatus and a video camera loaded with this apparatus. More particularly, the invention relates to a solid-state image pick-up apparatus using a solid-state image pick-up device that is able to perform operations with the use of an electronic shutter. The invention also relates to a video camera loaded with this type of apparatus.
2. Description of the Related Art
Some solid-state image pick-up devices execute electrical iris control for adjusting the exposure time according to the brightness of a subject. This iris control is exercised by the use of an electronic shutter that can vary the accumulation time (exposure time) of signal charges in a sensor unit. The basic principle of this control is that signal charges are caused to accumulate for a desired period immediately before reading the signal charges (optical charges) in the sensor unit and the charges accumulated before this period are swept away to a different place (for example, a substrate). FIG. 7 is a timing chart for illustrating the operation of the electronic shutter. Signal charges are read into the vertical transfer registers from the sensor unit at a timing in synchronization with the vertical synchronizing signal VD. A certain period (exposure timing) before the reading timing, a shutter pulse is applied to the substrate so that the signal charges accumulated in the sensor unit before this period can be swept away to, for example, the substrate. The period from the exposure timing until the reading timing is determined as an exposure period (exposure time).
The conventional solid-state image pick-up apparatus using the above type of the image pick-up device presents no problem when it is used under a light source providing constant illumination with the same level of brightness. However, the following problems are encountered by this apparatus when the apparatus is used under a light source, such as a fluorescent lamp, having periodic luminous characteristics and out of synchronization with the exposure cycle of the solid-state image pick-up device. For example, when a solid-state image pick-up device that performs exposure at 60 Hz is used in a combination with a fluorescent lamp emitting light at 100 Hz, flicker occurs, as shown in FIG. 8. More specifically, since the luminous cycle of the fluorescent light is 10 ms, and one cycle of the exposure operation performed at 60 Hz is 16.7 ms, the least common multiple results in 50 ms. As is seen from FIG. 9, after the exposure operation has been performed three times, the relationship between the two cycles returns to the original state in which the operation has started. Thus, there are three types of relationships of the exposure periods to the luminous cycle, and a significant difference in the output signal level of the image pick-up device is exhibited between the exposure periods. This causes flicker (luminance flicker) at 20 Hz.
This luminance flicker is intensified as the shutter speed increases, thereby seriously deteriorates the image quality, since flicker appears on the screen. It is known that 1/100-second electronic shutter is effective for inhibiting flicker when a solid-state image pick-up device (NTSC/EIA) performing exposure at 60 Hz is used in a combination with a fluorescent lamp emitting light at 100 Hz. However, this makes it impossible to perform exposure control by use of the electronic shutter. There is, in general, a significant difference in the color temperature of the fluorescent lamp depending on the luminous timing. Namely, the fluorescent lamp emits light with different color temperatures during the respective exposure periods for which signals A, B and C shown in FIG. 8 are obtained. This causes flicker of chroma signals (chroma flicker) in a color system, generating an image having the different levels of white balance between the fields and seriously degrading the image quality. This originates from due to the frequency of the exposure operation being lower than the luminous frequency of the fluorescent lamp, causing the aliasing component to emerge in the lower frequency band.